IOCG and Porphyry-Cu deposits in Northern Finland ... - IAGS 2011

Group in the Gällivare area is also considered to be
equivalent to the Sammakkovaara Group in the Pajala
area and is dominated by intermediate volcaniclastic
rocks and epiclastic sediments. In the Kiruna
area, these volcanic and sedimentary units are overlain
by the Kiirunavaara Group that is followed by
the Hauki and Maattavaara quartzites constituting
the uppermost Svecofennian units in the area.
In northern Finland, pelitic rocks in the
Lapland Granulite Belt were deposited after 1.94
Ga (Tuisku & Huhma 2006). Svecofennian units
are mainly represented by the Kumpu Group in the
CLGB (Lehtonen et al. 1998) and by the Paakkola
Group in the Peräpohja area (Perttunen & Vaasjoki
2001). The molasse-like conglomerates and quartzites
comprising the Kumpu Group were deposited in
deltaic and fluvial fan environments after 1913 Ma and
before c. 1800 Ma (Rastas et al. 2001). The Kumpu
rocks apparently are equivalent to the Hauki and
Maattavaara quartzites, and Porphyrite Group rocks
and the Kurravaara Conglomerate of the Kiruna area.
With the present knowledge of ages and petrochemistry
of the Porphyrite and Kumpu Groups,
it is possible to attribute these rocks completely to
the same event of collisional tectonics and juvenile
convergent margin magmatism. This period of convergence
was manifested by the numerous intrusions
of Jörn- (south of the craton margin) and Haparanda-
(within the craton) type calc-alkaline intrusions, as
described by Mellqvist et al. (2003). Within a few
million years, this period of convergent margin magmatism
was followed by a rapid uplift recorded in
extensive conglomeratic units, more alkaline and
terrestrial volcanism (Vargfors-Arvidsjaur Groups
south of the craton margin and the Kiirunavaara
Group within the craton) and plutonism (Gallejaur-
Arvidsjaur type south of the craton margin, Perthite
Monzonite Suite within the craton). This took place
between 1.88 and 1.86 Ga and the main volcanic
episode probably lasted less than 10 million years.
The evolution after c. 1.86 is mainly recorded
by an extensive S-type magmatism (c. 1.85 Ga
Jyryjoki, and 1.81–1.78 Ga Lina-type and the Central
Lapland Granitoid Complex) derived from anatectic
melts in the middle crust. In the western part of the
shield, extensive I- to A-type magmatism (Revsund-
Sorsele type) formed roughly N-S trending batholiths
(the Transcandinavian Igneous Belt) coeval with the
S-type magmatism. Scattered intrusions of this type
and age also occur further east (e.g. Edefors in Sweden,
Nattanen in Finland). The period from c. 1.87
to 1.80 Ga possibly also involved a shift in orogenic
vergence from NE-SW to E-W in the northern part
of the Shield as suggested by Weihed et al. (2002).
Palaeoproterozoic magmatism
Early rifting and emplacement
of layered igneous complexes
The beginning of the rifting period between 2.51
and 2.43 Ga is indicated by intrusion of numerous
layered mafic igneous complexes (Alapieti et
al. 1990, Weihed et al. 2005). Most of the intrusions
are located along the margin of the Archaean
granitoid area, either at the boundary against the
Proterozoic supracrustal sequence, totally enclosed
by Archaean granitoid, or enclosed by a Proterozoic
supracrustal sequence. Most of the intrusions
are found in W-trending Tornio-Näränkävaara belt
of layered intrusions (Iljina & Hanski 2005). Rest
of the intrusions are found in NW Russia, central
Finnish Lapland and NW Finland. These Palaeoproterozoic
layered intrusions are characteristic to
northern Finland as only one of them, the Tornio
intrusion, being partly on the Swedish side of the
border. Alapieti and Lahtinen (2002) divided the
intrusions into three types, (1) ultramafic–mafic, (2)
mafic and (3) intermediate megacyclic. They also interpret
the ultramafic–mafic and the lowermost part
of the megacyclic type to have crystallised from a
similar, quite primitive magma type, which is characterised
by slightly negative initial e Nd values and
relatively high MgO and Cr, intermediate SiO 2, and
low TiO 2 concentrations, resembling the boninitic
magma type. The upper parts of megacyclic type
intrusions and most mafic intrusions crystallised
from an evolved Ti-poor, Al-rich basaltic magma.
Amelin et al. (1995) suggested two age
groups for the intrusions for Fennoscandian Shield,
the first with U–Pb ages at 2.505–2.501 Ga, and the
second at 2.449–2.430 Ga. All Finnish layered intrusions
belong to the younger age group. The intrusions
were later deformed and metamorphosed during
the Svecofennian orogeny.
Mafic dykes
Mafic dykes are locally abundant and show a variable
strike, degree of alteration and metamorphic
recrystallisation which, with age dating, indicate
multiple igneous episodes. Albite diabase (a term
commonly used in Finland and Sweden for any albitised
dolerite) is a characteristic type of intrusions
that form up to 200 m thick sills. They have a coarsegrained
central part dominated by albitic plagioclase
and constitute laterally extensive, highly magnetic
units north of Kiruna.
Extensive dyke swarms occur in the Archaean
domain north of Kiruna; the swarms are dominated
by 1–100 m wide dykes with a metamorphic mineral
assemblage but with a more or less preserved
igneous texture (Ödman 1957, Martinsson 1999a,b).
The NNE-trending dykes that are suggested to represent
feeders to the Kiruna Greenstone Group (Mar-
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